The power supply system of electrified rail transport in the world is divided into two ways: DC power supply and AC power supply. The trunk line rail transport (also called large railway) adopting electrified lines has both DC power supply mode and AC power supply mode. The urban rail transport (including Metro and light rail) with electrified lines adopts DC power supply mode. For the DC power supply system of trunk rail transport, the power supply voltage is generally 3000V, and for urban rail transit's, the power supply voltage is generally 1500V or 750V. Two kinds of voltage system for DC power supply system of urban rail transport exist in our country. In cities like Beijing and Tianjin that built metro in the early days, the power supply system of many urban rail transport line is DC 750V. While in cities like Shanghai and Guangzhou that built metro in late days, the power supply system of urban rail transport line is DC 1500V. For rail transport line with DC supply voltage of 1500V or 3000V, overhead contact line is often used as supply line for train's power supply. While for rail transport line with DC supply voltage of 750V, electric contact rail (because a train usually needs two running rails, it is also called “third rail”) is often used as supply line for train's power supply.
With the economic development and social progress, the problem of traffic congestion in large and medium-sized city all over the world are becoming more and more serious. The construction of urban rail transport, such as metro or light rail, has become one of the most effective ways to solve the problem. At present, more than 30 cities in our country have begun to build urban rail transport. By the end of 2014, there were 88 urban rail transport systems operating in 21 cities, with a total mileage of 2787.25 km.
No matter trunk line rail transport or urban rail transport, as long as the use of DC power supply system, so far, in addition that the urban transport in Singapore adopts the fourth rail as the backflow line, in other countries and regions, basically the running rail of train is used as the backflow line (running rail backflow line. The following problems often occur in the DC power supply system using the running rail return line:    (1) The traction backflow current of DC traction substation returned from the train will enter the ballast bed through the bad area where the running rail is poorly insulated, forming stray current. Stray current can cause electrochemical corrosion to the running rail, the reinforcing steel of the whole ballast bed, structural reinforcement of tunnel, the reinforcement of the bridge and the metal equipment along the urban rail, and then affects the service life of the building structure and metal equipment along urban rail transport.    (2) Due to the relatively large resistivity of the steel material of the running rail, there exists a problem that ground potential of running rail is too high when running through a large current, which leads to frequent action of the potential limiter of the running rail.
In addition, in the DC power supply system of the rail transport, the regenerative braking energy is not fully utilized. A large number of regenerative braking energy is consumed by the braking resistor in the form of heating, which not only causes great waste, but also brings about the temperature rise of the tunnel when the underground line is running Therefore, Regenerative braking energy is recycled by energy feedback device or super capacitor energy storage device, which is the main method to solve this problem. But the energy feedback device (energy feedback grid) involves many engineering problems, and the super capacitor energy storage device occupies too much space.